This application claims the benefit of Japanese Patent Application Number 2014-081310 filed on Apr. 10, 2014, the entirety of which is incorporated by reference.
Technical Field
The present invention relates to a measurement system for measuring and correcting geometric errors in a multi-axis machine tool.
Background Art
In the field of machine tools, to achieve highly efficient machining and to machine a workpiece having a complicated shape, multi-axis machine tools, such as a 5-axis machining center which includes a conventional 3-axis machining center with two rotary axes added, have been developed, and improvement of machining accuracy thereof have been expected. In general, assembling (manufacturing) thereof becomes more difficult and machining accuracy tends to be deteriorated as the number of axes increases, and there is a limit on improvement of machining accuracy through assembling. Thus, a correction system has been developed to improve machining accuracy by correcting so-called geometric errors such as the slant and positional errors between adjacent axes.
As a method for identifying geometric errors in the aforementioned correction system, there is known a method to obtain geometric errors from measurement results by using measuring instruments such as displacement gauge and right-angled square. The method described above, however, requires several measuring instruments, and identification accuracy using the method varies largely caused by uncertain measurement results due to different measuring skills or the like with the method. As a method for identifying geometric errors in the 5-axis machining center, there is also widely known measurement of simultaneous 3-axis arc interpolation motion using a displacement sensor called a ball bar (i.e. method for identifying geometric errors from the amount of deviation at the center of an circular trajectory that is obtained by synchronizing two linear axes and one rotary axis and causing a circular motion of a ball bar so as to maintain relative displacement between one point and a main spindle on a table). However, the method requires a special measuring instrument, for example, a ball bar and identification accuracy varies depending on the setting of the ball bar.
Therefore, a measurement system as shown in Japanese Patent Application Publication No. 2011-038902 (JP 2011-038902 A) was developed to obtain geometric errors by using a touch probe (which is often mounted on machine tools) and a sphere serving as a target (target sphere), based on the same principle as the measurement of simultaneous 3-axis arc interpolation motion using a ball bar. In the measurement system, for example, to obtain geometric errors at a trunnion table type 5-axis machining center, a target sphere is set on the table initially to measure established coordinates. Afterwards, various angles are indexed for rotary axes, and center coordinates and a diameter of the target sphere are measured under each indexing condition. Then, the amount of deviation at the center of an arc trajectory drawn by these plural coordinates of the target sphere is calculated to identify a geometric error. Besides, when geometric errors is identified by the measurement system, center coordinates and a diameter of the target sphere positioned by indexing the rotary axes are measured by using three linear axes. For example, the touch probe is brought into contact with the target sphere by a motion made by only one certain axis to the target sphere, more specifically, by a uniaxial motion from +X direction, −X direction, +Y direction, −Y direction and +Z direction in order to calculate center coordinates and a diameter of the target sphere from coordinates of four or more contact points.